1. Field of the Invention
The present invention relates to an aberration correction apparatus and a charged particle apparatus, and more particularly to an optical system for correcting a spherical aberration of an objective lens, of a charged particle beam microscope typified by a transmission electron microscope with, for example, an aberration correction apparatus including multipole lenses and rotationally symmetric lenses.
2. Background Art
An electron lens that uses an electric field or a magnetic field to converge electron beams is essential in an electron microscope such as a scanning electron microscope (SEM), a transmission electron microscope (TEM), or a scanning transmission electron microscope (STEM). A lens that acts as a spherical lens using a rotationally symmetric electromagnetic field is most often used as an electron lens. It is known that a positive spherical aberration is unavoidable with this kind of rotationally symmetric electron lens. In this case, since it is not possible to create a negative spherical aberration even by using another rotationally symmetric electron lens, spherical aberration correction can not be performed by using a set of concave/convex lenses in optics, and in the conventional electron microscope apparatuses spherical aberration is a principal factor in deciding the substantial resolution.
Meanwhile, it has been indicated that the spherical aberration of an electron lens can be corrected in theory by a combination of non-rotationally symmetric multipole lenses. However, the structure of these multipole correctors is complex since the structures use multiple stages of four poles, six poles, eight poles, or the like.
One known aberration correction apparatus corrects the spherical aberration of a rotationally symmetric lens by generating a hexapole field with a multipole lens. The principle of the spherical aberration correction is as follows. With respect to a positive spherical aberration of a common objective lens, a negative spherical aberration is produced by generating a hexapole field with a multipole lens to thereby cancel out the spherical aberration of the objective lens. Further, since the negative spherical aberration can be controlled by changing the strength of the hexapole field, the spherical aberration of the lenses mounted in the electron microscope, that is, the entire optical system including an objective lens, converging lens, projection lens and the like, can be controlled to an arbitrary amount. However, the hexapole field generates a secondary aberration, and therefore the secondary aberration of the hexapole field can be cancelled out by arranging two rotationally symmetric lenses between two multipole lenses and inverting the trajectory of the electron beam between the multipole lenses.
For example, technology disclosed in JP Patent Publication (Kokai) No. 3-295140 A (1991) (hereunder, referred to as “Patent Document 1”) relates to an apparatus that corrects the spherical aberration of a rotationally symmetric lens of an electron microscope of this kind. FIG. 1 is a schematic diagram that illustrates that technology. In FIG. 1, although each electron lens is illustrated as an optical lens, this is done to simplify the diagrammatic representation, and in fact the lenses are electron lenses that use a magnetic field.
In the correction apparatus, rotationally symmetric lenses 4 are 5 are arranged between multipole lenses 2 and 3, and rotationally symmetric lenses 7 and 8 are arranged between a multipole lens 2 and an objective lens 6. The focal lengths of the rotationally symmetric lens are all the same length f. A distance between the rotationally symmetric lenses 7 and 8 is 2 f, and a distance from the multipole lens 2 to each of the rotationally symmetric lenses 4 and 8 is f. Further, a distance between the rotationally symmetric lenses 4 and 5 is 2 f, and a distance between the multipole lens 3 and the rotationally symmetric lens 5 is f. Conventionally, when performing high resolution observation with an electron microscope, a specimen position 9 exists inside an objective lens 6, and an extremely strong excitation of several mm is used as the focal length of the objective lens 6. An axial trajectory 10 is an electron beam trajectory that passes an intersection point between the specimen and the optical axis and that has a certain angle with respect to the optical axis, and is incident onto the multipole lens 2 in parallel with the optical axis. Thereafter, the trajectory is inverted by the rotationally symmetric lenses 4 and 5, and is incident on the multipole lens 3 in parallel with the optical axis (spherical aberration correction condition). When the multipole lenses 2 and 3 and the rotationally symmetric lenses 4 and 5 are arranged as described above, the axial trajectory 10 passes through the multipole lenses 2 and 3 in a condition in which the axial trajectory 10 is separated by the same distance from the optical axis (spherical aberration correction condition). Since the strength of a hexapole field is decided by the distance from the optical axis, a secondary aberration can be cancelled out by making the excitation of the two multipole lenses the same. More specifically, by applying a spherical aberration of opposite sign and half the amount with respect to the spherical aberration of the objective lens 6 with the respective hexapole fields of the two multipole lenses, the spherical aberration of the objective lens can be corrected while cancelling out the secondary aberration.
The aberration correction apparatus shown in FIG. 1 has a configuration for correcting an axial coma aberration. A rotationally symmetric lens has a plane without an axial coma aberration, and the plane is called a “coma-free plane”. Since the coma-free plane normally exists in the vicinity of a back focal plane of the rotationally symmetric lens, in a case in which the specimen position 9 is arranged inside the objective lens 6 and high resolution observation of the objective lens 6 is performed using a strong excitation, a coma-free plane 11 of the objective lens 6 exists at a position that is several mm behind the objective lens 6. Assuming that the distance between the coma-free plane 11 of the objective lens 6 and the rotationally symmetric lens 7 is f, the coma-free plane 11 can be transferred to the coma-free plane of the rotationally symmetric lens 7.
According to the configuration shown in FIG. 1, the coma-free plane 11 can be transferred to the coma-free plane of the rotationally symmetric lenses 8, 4, and 5 according to the same principle. The coma aberration of a multipole lens has trajectory that passes through the center of the multipole lens, and the coma aberration can be cancelled out by the trajectory becoming symmetrical at the center between the two multipole lenses 2 and 3. In FIG. 1, an off-axis trajectory 12 that passes through the coma-free plane 11 of the objective lens 6 passes through the center of the multipole lenses 2 and 3, and by the trajectory being made symmetrical at the center between the two multipole lenses 2 and 3, the coma-free plane is transferred to correct the axial coma aberration.
According to the configuration of FIG. 1 described above, a spherical aberration correction apparatus is provided in which a spherical aberration correction condition (beam is incident in parallel with the optical axis with respect to the multipole lenses 2 and 3, and the beam is symmetrical (distance from optical axis is the same) at multipole lenses 2 and 3) is satisfied by the axial trajectory 10 between the multipole lenses 2 and 3 that are the latter half portion of the correction apparatus, and a coma-free plane transfer condition is satisfied by the off-axis trajectory 12 between the objective lens 6 and the multipole lens 2 that are the front half portion of the correction apparatus.
JP Patent Publication (Kohyo) No. 2002-510431 A (hereunder, referred to as “Patent Document 2”) discloses technology for correcting a spherical aberration using a different configuration. FIG. 2 is a schematic diagram that illustrates that technology. The apparatus in FIG. 2 is also an aberration correction apparatus for high resolution observation in which the specimen position 9 is inside the objective lens 6, similarly to FIG. 1. Although the configuration between the multipole lenses 2 and 3 as the latter half portion of the correction apparatus is the same as that shown in FIG. 1, the configuration between the objective lens 6 and the multipole lens 2 as the front half portion of the correction apparatus is different to that shown in FIG. 1.
In FIG. 2, the focal lengths of the rotationally symmetric lenses 7 and 8 are assumed to be f1 and f2, respectively. The distance between the coma-free plane 11 of the objective lens 6 and the rotationally symmetric lens 7 is assumed to be f1, the distance between the rotationally symmetric lenses 7 and 8 is assumed to be f1+f2, and the distance from the rotationally symmetric lens 8 to the multipole lens 2 is assumed to be f2. Since the latter half portion of the correction apparatus has the same configuration as that in FIG. 1 described above, the axial trajectory 10 satisfies the spherical aberration correction condition by the same principle. Further, although the configuration of the front half portion of the correction apparatus is different to the configuration shown in FIG. 1, by disposing the rotationally symmetric lenses 7 and 8 at the positions of the focal lengths, the off-axis trajectory 12 satisfies the coma-free plane transfer condition.
As a feature of the aberration correction apparatus shown in FIG. 2, the ease with which a spherical aberration can be finely adjusted may be mentioned. Since the spherical aberration and coma-free plane 11 of the objective lens 6 change when the focal length of the objective lens 6 is changed, according to the configuration shown in FIG. 1 it is necessary to adjust the focal length f of all the rotationally symmetric lenses. However, with the configuration shown in FIG. 2, when finely adjusting a spherical aberration correction of the objective lens 6, since the position of the axial trajectory 10 passing through the multipole lens 2 does not change when focal lengths f1 and f2 of the rotationally symmetric lenses 7 and 8 are finely adjusted, the spherical aberration can be corrected without changing the focal length f of the rotationally symmetric lenses 4 and 5 and the excitation of the multipole lenses 2 and 3. Hence, according to the configuration shown in FIG. 2, the spherical aberration correction condition and coma-free plane transfer condition can be satisfied by finely adjusting the positions and focal lengths f1 and f2 of the rotationally symmetric lenses 7 and 8, and the advantage that fine adjustment of a spherical aberration correction can be performed with ease can be expected. Further, since the focal length can be adjusted according to the configuration shown in FIG. 2, in comparison to the configuration shown in FIG. 1, the configuration shown in FIG. 2 allows greater flexibility and, for example, also enables magnification of an image.